Heat exchangers such as radiators, condensers, and evaporators to be mounted in automobiles are generally manufactured by forming, assembling, and brazing aluminum alloy sheets, which are lightweight and excel in heat conductivity. Most of aluminum alloy core materials for use in such heat exchangers adopt Al—Mn alloys having relatively high strengths. In addition, recently developed core materials further contain alloy elements such as Cu, Si, and Mg so as to have further higher strengths.
When such an aluminum alloy material for heat exchangers is used as a tube typically in a radiator, the outer surface of the tube is exposed to the atmosphere (air), and the inner surface is exposed to a coolant such as cooling water. The tube, when exposed to such a corrosive environment, may suffer from corrosion (pitting corrosion) proceeding locally, resulting in the generation of through holes (penetrating holes). For preventing corrosion of the outer surface of the tube, so-called “sacrificial protection” is generally employed and effective, in which a fin material typically containing an Al—Zn alloy or another substance having a less-noble potential than that of the aluminum alloy constituting the tube is brought in contact with the tube. Also for preventing corrosion of the inner surface of the tube, the sacrificial protection technique is often employed. Specifically, in this case, the tube is generally formed from a clad material including an aluminum alloy core material, and clad on the inner side of the core material, a sacrificial anode material (hereinafter also referred to as a “sacrificial material”) of an Al—Zn alloy having a less-noble potential than that of the aluminum alloy of the core material. The outer surface of the tube is often clad with an Al—Si alloy or another brazing material having a low melting point, for the purpose of brazing typically with the fin material.
As is described above, clad materials including three or more layers and having a core material (core layer), and clad thereon, a sacrificial material (sacrificial layer) and a brazing material (brazing layer) are often used as aluminum alloy materials for heat exchangers.
Increasing demands have been made on such aluminum alloys for heat exchangers to have longer lives and smaller thicknesses (lighter weights) and to have further higher corrosion resistance. Exemplary techniques for further improving the corrosion resistance of aluminum alloys for heat exchangers include those disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. 2009-228010 and Japanese Unexamined Patent Application Publication (JP-A) No. 2008-231555. JP-A No. 2009-228010 discloses an aluminum alloy brazing sheet in which compositions of the core material and cladding are controlled. JP-A No. 2008-231555 discloses an aluminum alloy composite material in which the composition of the core material is controlled, and the distribution of Al—Mn intermetallic compounds is also controlled.